It is well known that an aromatic primary amine color developing agent oxidized by exposed silver halide as an oxidizing agent reacts with a coupler to form a color image.
Magenta color image forming couplers which have been widely used in practice and on which various investigations have been made are almost always 5-pyrazolones. However, it is known that dyes formed from 5-pyrazolone type couplers have an undesirable absorption of the yellow component in the region around 430 nm, which causes color turbidity.
In order to reduce the yellow component absorption, a pyrazolobenzimidazole nucleus as described in British Patent 1,047,612, an indazolone nucleus as described in U.S. Pat. No. 3,770,447, a pyrazolo[5,1-c]-1,2,4-triazole nucleus as described in U.S. Pat. No. 3,725,067, a pyrazolo[1,5-b]-1,2,4-triazole nucleus as described in JP-A-59-171956 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and U.S. Pat. No. 4,540,654 and an imidazo[1,2-b]pyrazole nucleus as described in JP-A-59-162548 and U.S. Pat. No. 4,500,630 have been proposed as a magenta color image forming coupler skeleton.
Among them, pyrazolo[5,1-c]-1,2,4-triazole type magenta couplers and pyrazolo[1,5-b]-1,2,4-triazole type magenta couplers (both types of couplers are generally referred to as pyrazolotriazole type couplers hereinafter) are particularly useful and have been practically employed in some silver halide color photographic materials.
Dyes formed from the pyrazolotriazole type couplers are superior to those formed from conventional 5-pyrazolones in view of the undesirable absorption of the yellow component and they are also preferred from a standpoint of color reproduction since their absorption spectra sharply decrease to zero on the longer wavelength side.
The inventors have found, however, that these pyrazolotriazole type couplers have the undesirable properties described below, although they have the above described excellent properties. When these couplers are used together with silver halide, which acts as an oxidizing agent for an aromatic primary amine developing agent necessary for a color forming reaction, and more specifically, when an emulsified dispersion of the couplers and a silver halide emulsion are mixed to form a coating and its photographic characteristics are evaluated, the silver halide emulsion does not have its original sensitivity, gradation or fog, and as a result the sensitivity may increase or decrease depending on the emulsion used and the resulting color density decreases. Such facts can be determined by a comparison of these results with those obtained by color development or black and white development of a coating prepared by incorporating a 5-pyrazolone type coupler under the same conditions. Also, some of these facts can be confirmed by a comparison with those results obtained by black and white development of a coating containing only the silver halide emulsion.
It is not essentially expected that the couplers used in silver halide color photographic materials have any effect on the silver halide emulsions used except for couplers having a specific function such as development inhibitor releasing couplers or developing accelerator releasing couplers. Particularly, it is undesirable to have an effect on a light sensitive process and cause sensitization or desensitization. With recent silver halide photographic materials in which rapid processing capability is required, it is strongly desired not to influence the silver halide emulsion.
The inventors have confirmed that pyrazolotriazole type couplers have stronger interaction with silver ion or silver halide in comparison with compounds having a 5-pyrazolone residue which have hitherto been widely used as magenta couplers, specifically to effect the formation of complex or adsorption. Further, it has been confirmed that such an interaction causes desensitization of the emulsion and a decrease in color reproduction.
The inventors have determined that the undesirable interaction of a pyrazolotriazole type coupler with silver is difficult to control and turned his attention to an imidazopyrazole nucleus for solving the problem. However, dyes formed from compounds having an imidazopyrazole nucleus are apt to have their spectral absorption in a shorter wave length region as compared with those formed from pyrazolotriazole type couplers and they are not preferred in view of color reproduction.